In lithography, apodizing masks modify the intensity of light, both locally and globally. Apodizing masks can be constructed through a number of processes. Photo-electro forming is one process for producing apodizing masks involving electro-deposition of metal to build-up a film with no glass substrate.
The performance of apodizing masks is largely defined by the ratio of maximum to minimum light transmission. Such ratio is limited in electro-deposited apodizing mask films because of the minimum feature resolution size. For example; the minimum feature resolution size in an electro-deposition process is 1 μm. At the scale of apodizing masks, diffraction is a significant consideration; ideally, during regular use, only the zeroth order diffraction will be received by a charge-coupled device (CCD), CMOS, or any other type of image sensor. In common wavelengths and for common image sensors, the maximum pitch is on the order of 9 μm; a pitch greater than 9 μm results in first diffraction orders encroaching on the CCD. Therefore, in such an example, the maximum transmission percentage is approximately 88% while the minimum transmission percentage is approximately 11%, corresponding to an 8-to-1 light transmission ratio.
Consequently, it would be advantageous if an apparatus existed that is suitable for producing an apodizing mask film having an improved light transmission ratio. Specifically, an improved light transmission ratio may be understood as a higher light transmission ratio (>88%).